An absorbent core containing a hydrophilic fiber such as pulp and a water absorbent resin as its components is widely used in sanitary materials such as disposable diapers, sanitary napkins, incontinence pads and the like, in order to absorb body fluids.
Recently, the sanitary material such as the sanitary napkin and the like has higher performance and a thinner size, and an amount of the water absorbent resin used for each sanitary material tends to increase, and also a ratio of the water absorbent resin tends to increase with respect to the whole absorbent core constituted of the water absorbent resin and a hydrophilic fiber. That is, by using (i) a smaller amount of a hydrophilic fiber whose bulk density is low and (ii) a larger amount of a water absorbent resin having a superior water absorbing property and high bulk density, a ratio of the water absorbent resin contained in the absorbent core is increased, thereby making the sanitary material thinner without decreasing an amount of water absorption.
However, the sanitary material which includes a smaller amount of the hydrophilic fiber and a larger amount of the water absorbent resin is preferable merely in terms of liquid storage, but raises problems in terms of distribution and diffusion of liquid in actual use in diapers. For example, when a large amount of the water absorbent resin is used, the water absorbent resin becomes soft and gelatinous upon absorbing water. This causes a gel blocking phenomenon. As a result, a liquid diffusing property of the diaper significantly drops. In order to avoid such phenomenon and to keep high absorbing property of the absorbent core, a ratio of the hydrophilic fiber and the water absorbent resin is inevitably limited, so that there is a limit in making the sanitary material thinner.
As means for obtaining a water absorbent resin which is superior in liquid permeability and liquid diffusing property while preventing the gel blocking, a technique in which a metal component (metal salt, metal cation, or the like) is added to the water absorbent resin is known (for example, see Japanese Unexamined Patent Publication No. 7745/1987 (Tokukaisho 62-7745), Japanese Unexamined Patent Publication No. 270741/1988 (Tokukaisho 63-270741), Japanese Unexamined Patent Publication No. 124879/1997 (Tokukaihei 9-124879), U.S. Pat. No. 6,323,252, and International Publication No. 01/74913 pamphlet).
According to such a technique, a metal component (metal salt, metal cation, or the like) is added as an aqueous solution, so that a metal component permeates the water absorbent resin. Thus, it is impossible to sufficiently improve the liquid, permeability and liquid diffusing property so as to correspond to an amount of the metal component added. Further, the metal components permeate the water absorbent resin, so that an absorbency without load and an absorbency against pressure drop. Moreover, according to such a technique, the metal components unevenly adhere to a surface of the water absorbent resin in a plane manner, so that there is a problem in terms of expression of even properties.